Field
The disclosed technology relates to methods for patterning amorphous semiconductor layers, such as amorphous silicon layers.
Methods in accordance with the disclosed technology may be used for patterning amorphous silicon layers in a fabrication process of silicon heterojunction photovoltaic cells, such as silicon heterojunction interdigitated back contact cells.
The project leading to this application has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 657270.
Description of the Related Technology
In a method for fabricating silicon heterojunction (SHJ) photovoltaic cells, a heterojunction may be formed by providing a thin doped hydrogenated amorphous silicon (a-Si:H) layer on a crystalline silicon substrate. A thin intrinsic hydrogenated amorphous silicon layer may be provided on the crystalline silicon substrate surface before providing the doped hydrogenated amorphous silicon layer, for good surface passivation. In a fabrication process of SHJ interdigitated back contact (SHJ-IBC) photovoltaic cells, there is a need for providing differently doped regions at the rear side. This means that at the rear side of the crystalline silicon substrate a patterned p-type a-Si:H layer and a patterned n-type a-Si:H layer are to be formed side by side. Typically first an a-Si:H layer of a first doping type is formed on the entire rear side and this layer is patterned by locally removing the layer according to a predetermined pattern. Afterwards an a-Si:H layer of a second, opposite, doping type is provided at locations where the first a-Si:H layer of the first doping type has been removed. This requires re-passivation of the crystalline silicon surface at those locations.
Photolithography may be used for patterning an a-Si:H layer, but this is not compatible with large-scale industrial production. As an alternative to photolithography laser ablation may be used. However, during the laser ablation process the crystalline silicon surface is damaged. The laser ablation process may induce defects, surface roughness and/or thermal damage to the silicon substrate, such that there is a need for performing a laser damage removal step after patterning of the a-Si:H layer and a good re-passivation of the crystalline silicon surface in the ablated regions may be problematic. Therefore, methods are being developed wherein laser ablation induced damage is reduced or substantially avoided.
In “SLASH Concept: A Novel Approach for Simplified Interdigitated Back Contact Solar Cells Fabrication”, Proceedings of the 38th IEEE PVSC, 2012, pages 001602-001605, T. Desrues et al. propose a method wherein a PECVD (Plasma Enhanced Chemical Vapor Deposition) SiNx:H layer is provided on top of an a-Si:H layer to be patterned. The PECVD SiNx:H layer is first patterned by laser ablation, and next the underlying a-Si:H layer is patterned by wet chemical etching using the patterned SiNx:H layer as a dielectric hard mask. By optimizing the deposition conditions of the SiNx:H layer its absorption coefficient at the laser wavelength is increased/maximized, such that it absorbs a substantial part of the laser power and less laser power can reach the underlying a-Si:H layer and crystalline silicon substrate. This results in a reduction of laser damage to the a-Si:H layer and the crystalline silicon substrate as compared to a method wherein the a-Si:H layer is directly laser ablated. However, when using this method for forming a pattern comprising a continuous shape such as a line shape or a rectangular shape, as for example required in a process for fabricating SHJ-IBC cells, the pattern is formed by providing laser pulses in adjacent dot-shaped areas. In such a process, an overlap between two adjacent laser pulses or two adjacent dot-shaped areas cannot be avoided. This results in an increased laser power in the overlapping areas, reaching the underlying a-Si:H layer and crystalline substrate and inducing damage.
In “Emitter Patterning for Back-Contacted Si Heterojunction Solar Cells Using Laser Written Mask Layers for Etching and Self-Aligned Passivation (LEAP)”, IEEE Journal of Photovoltaics, Vol. 6, No. 4, July 2016, pages 894-899, Sven Ring et al. describe a process that allows reducing or substantially avoiding laser damage to the substrate in areas of overlapping laser pulses. This method comprises: depositing, on top of a crystalline substrate with an a-Si emitter layer to be patterned, a SiO2/a-Si:H/SiO2/a-Si:H layer stack; patterning the upper a-Si:H layer by laser ablation; patterning the underlying SiO2 and a-Si:H layers by wet etching in HF and KOH respectively using the laser ablated a-Si:H layer as an etch mask; opening the underlying SiO2 layer using wet etching in HF with the lower a-Si:H interlayer acting as an etch mask; and finally etching the a-Si emitter layer in an alkaline etchant. It is shown that the lower a-Si:H interlayer of the SiO2/a-Si:H/SiO2/a-Si:H layer stack may protect the substrate from laser damage during laser ablation of the upper a-Si:H layer of the stack. It is a disadvantage of this approach that patterning of the SiO2 and a-Si:H layers underlying the upper a-Si:H layer, after laser ablation of the upper a-Si:H layer, requires different etchants for the different layers, e.g. an HF based etchant for patterning the SiO2 layers and an alkaline etchant for patterning the lower a-Si:H layer.